Road Users Charges
Review Group
ENGINEERING ADVICE
„
23 February 2009
Road Users Charges Review Group
ENGINEERING ADVICE
„
23 February 2009
Sinclair Knight Merz
ABN 37 001 024 095
100 Christie Street
PO Box 164
St Leonards NSW
Australia 1590
Tel: +61 2 9928 2100
Fax: +61 2 9928 2500
Web: www.skmconsulting.com
COPYRIGHT: The concepts and information contained in this document are the property of Sinclair
Knight Merz Pty Ltd. Use or copying of this document in whole or in part without the written
permission of Sinclair Knight Merz constitutes an infringement of copyright.
LIMITATION: This report has been prepared on behalf of and for the exclusive use of Sinclair
Knight Merz Pty Ltd’s Client, and is subject to and issued in connection with the provisions of the
agreement between Sinclair Knight Merz and its Client. Sinclair Knight Merz accepts no liability or
responsibility whatsoever for or in respect of any use of or reliance upon this report by any third
party.
The SKM logo trade mark is a registered trade mark of Sinclair Knight Merz Pty Ltd.
Road Users Charges Review Group
Engineering Advice
Sinclair Knight Merz (SKM)
Sinclair Knight Merz is an Australian based international Consulting Group with offices in
Wellington, Auckland and Christchurch.
The Group provides transport engineering and planning services to a wide range of international
clients including policy advice to government transport agencies.
Authors
This report was prepared by Gabriel Tooma and Peter Prince of SKM.
For further information on aspects of this report, contact the authors at [email protected] or
[email protected].
Disclaimer
The focus of this report is the engineering relationships in the Road User Cost (RUC) model
allocation to vehicle classes for the purpose of the cost recovery of New Zealand road use.
The content of this report is based on existing information and on the information and documents
made available to the authors. The independent views expressed are those of the authors.
The findings and conclusions do not necessarily represent the views of the New Zealand Ministry
of Transport or the New Zealand Transport Agency.
SINCLAIR KNIGHT MERZ
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Road Users Charges Review Group
Engineering Advice
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Road Users Charges Review Group
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Contents
Executive Summary
1 1. Introduction and Scope of Works
3 1.1. 3 2. 3. 4. 5. Introduction
The New Zealand Road User Cost (RUC) System
5 2.1. 6 Road costs and vehicle weight and axle loading
The Fourth Power Rule
3.1. 3.2. Introduction
Review of Recent NZ Research
3.2.1. 3.2.2. 3.2.3. 3.2.4. 3.2.5. Vertical Surface Deformation
End of life criteria
VSD Extrapolated to end of life
Subgrade strength
Summary comments on Report 281
3.3. Review of other relevant reports
3.3.1. 3.3.2. 3.3.3. 3.3.4. 3.3.5. 3.3.6. 3.3.7. 3.3.8. Ternz and Covec
Arnold et al.
Sharp and Vuong
Yeo, Martin and Koh
Chen et al
Johnsson 2004
De Pont
Austroads
3.4. Fourth Power Rule Conclusions
9 9 10 12 13 15 20 20 21 21 21 22 22 23 24 24 24 25 Vehicle Dynamics and Its Effect on Road Wear
27 4.1. 4.2. 4.3. 4.4. 27 27 28 28 RUC Model
Effect on Road wear
Other Views
Conclusion
Reference Loads
29 5.1. 5.2. 5.3. 5.4. 29 29 29 30 RUC Model
Effect on Road wear
Other Views
Conclusion
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6. 7. 8. 9. Vehicle Weight Distribution
31 6.1. 6.2. 6.3. 6.4. 31 31 31 31 RUC Model
Effect on Road wear
Other views
Conclusion
Vehicle Types
32 7.1. 7.2. 7.3. 7.4. RUC Model
Effect on Road wear
Other Views
Conclusion
32 32 32 32 Vehicle Loading and Utilisation
33 8.1. 8.2. 8.3. 8.4. 33 33 33 34 RUC Model
Effect on Road wear
Other views
Conclusion
Large Single tyres
35 9.1. 9.2. 9.3. 9.4. 35 35 35 36 RUC Model
Effect on Road wear
Other views
Conclusion
10. Diesel Excise
37 Appendix A List of References
41 SINCLAIR KNIGHT MERZ
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List of Tables
„
Table 1 - Axle Reference Loads For Standard Tyres (tonnes/axle)
„
Table 2 CAPTIF Test Segments and attributes
11
„
Table 3 LDE using Linear model for extrapolation to end of life
18
„
Table 4 LDE using Power Model for extrapolation to end of life
19
„
Table 5 Effect of subgrade on SNP and LDE
20
„
Table 6 LDE from various researchers
21
„
Table 7 LDE
23
„
Table 8 LDE from Austroads Pavement Design Guide
25
7
List of Figures
„
Figure 1 - Overview of NZ RUC
„
Figure 2 Comparison of VSD and rut measurement at CAPTIF
12
„
Figure 3 Linear and power model extrapolation methods compared to actual results
16
„
Figure 4 Extrapolation of VSD data using the power model
16
„
Figure 5 Extrapolation of VSD data using the linear model
17
„
Figure 6 Typical pavement structure
23
„
Figure 7 The fuel consumption of heavy duty vehicle as loads increase
37
„
Figure 8 RUC vs. Excise for Class 6
38
„
Figure 9 RUC Charges
39
5
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Document history and status
Revision
Date issued
Reviewed by
Approved by
Date approved
Revision type
01
17 Dec 2009
G Tooma
P Prince
16 Dec 2008
Draft 1
02
23 Feb 09
G Tooma
P Prince
23 Feb 09
Draft 2
Distribution of copies
Revision
Copy no
Quantity
Issued to
01
01
1
T Brennard
02
01
1
T Brennard
Printed:
24 February 2009
Last saved:
24 February 2009 11:02 AM
File name:
I:\NBIF\Projects\NB10990\Deliverables\NB10990 Draft Report R04 230209.doc
Author:
Gabriel Tooma and Peter Prince
Project manager:
Gabriel Tooma
Name of organisation:
New Zealand Ministry of Transport
Name of project:
Road Users Charges Review Group
Name of document:
Engineering Advice
Document version:
Final
Project number:
NB10990
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Executive Summary
Introduction
This report presents findings of an independent review of the engineering relationships within the
Road User Cost (RUC) model currently being investigated by the Road User Charges Review
Group. It has been prepared by consultants Sinclair Knight Merz (SKM), based on existing
technical reports and findings from relevant research work and reviews undertaken by others.
The main findings are presented below.
Fourth Power Rule
The fourth power rule, relating thin flexible pavement damage to vehicle axle loads, which forms
an important pavement wear cost within the RUC, continues to be used in road design in all
comparable and relevant countries, including Australia.
It represents a proven relationship applied to a wide range of pavement strengths and conditions.
There is no intention of State road authorities within the Austroads membership to change this
relationship. The latest Austroads research into cost allocation suggests flexible pavement damage
and changes should be based on a net tonne kilometre using the fourth power rule.
Evidence available suggests that road pavements on generally low trafficked and weak road
pavements are on average subject to higher damage than on national roads under the same axle
load.
The recent Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) research indicates
a lower power rule than four would more accurately represent equivalent pavement damage to the
point of pavement failure. The definition of failure (15mm deformation over 10% of the pavement
surface) used in the laboratory is far different from pavement conditions in practice.
A large part of the New Zealand network (National and Local roads) operates at levels of
deformation in excess of 15mm before renewal.
A lower power rule would greatly increase the risks of both higher pavement damage and
rehabilitation costs over the life of pavements across the network.
A change towards a lower road damage power would significantly put at risk the integrity of
the road network and the ability of government to afford the resulting pavement
rehabilitation upkeep programs especially in respect to local roads. Furthermore, such a
change would unfairly in our view, pass user fees from large vehicles to smaller vehicles. For
these reasons the power law should not be changed.
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Vehicle Dynamics
There is sufficient evidence (from Austroads tests) to support the case that air suspensions (called
road-friendly suspensions) reduce pavement deformation. Road-friendly suspensions should be
recognised in the RUC model provided vehicles with road-friendly suspensions are monitored
through a third party system.
Reference Loads
Reference loads in the RUC model should be aligned with Austroads and current road design
practice in New Zealand. A consistent approach should be taken.
Vehicle Types
New vehicle types should be investigated and included in the RUC model to better represent the
current vehicle fleet.
Vehicle Loading and Utilisation
In the absence of evidence and information on actual distances travelled, the current assumption
(55% of vehicles travelled at full load) should be maintained until new data is available.
Large Single Tyres
Single wide tyres should be recognised in the RUC model as vehicles fitted with these tyres
increase pavement stress compared with twin standard tyres. Vehicles fitted with wide single tyres
should have a different vehicle classification and corresponding RUC rates schedule.
Weight Distribution
Averaging out of ESA’s per axle as in the RUC model, disadvantages some vehicle types. The
effect of vehicle weight distribution on road pavement wear is not as significant however (in terms
of wear and tear costs) as the other above factors which should be addressed first.
Diesel Excise
A diesel excise is not related to pavement wear whatsoever.
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1.
Introduction and Scope of Works
1.1.
Introduction
Ministry of Transport, New Zealand (MoT) has engaged SKM to provide engineering advice to the
Road User Charges Review Group late November 2008.
Sinclair Knight Merz (SKM) understands that the Road User Review Group is an independent
committee, appointed by the Minister of Transport to undertake a review of the Ministry's road user
charges cost allocation model and of the method of collecting the proportion of transport network
costs attributable to diesel vehicles.
The Ministry specifically required in its brief the following topics to be addressed:
“
„
„
whether the assumptions about the impacts of vehicle weight and axle configuration in the
Cost Allocation Model appropriately reflect road engineering practice and road network
conditions in New Zealand. This will be informed by:
–
theoretical context on the nature of the relationship between road construction and
maintenance costs and vehicle weight and axle loadings; and in particular
–
an assessment of the relative merits of the different points of view on the relationship
between axle weight and road wear and the relationship between axle configuration and
road wear.
the extent of correlation between fuel consumption of heavy vehicles and their road network
costs.”
SKM has prepared this report to provide professional engineering opinion as required above in the
following chapters:
„
Chapter 2
Describes an outline of the RUC, for the purpose of investigating vehicle wear
effects.
„
Chapter 3
Discusses the fourth power rule.
„
Chapter 4
Briefly reviews the effect on road wear from vehicle dynamics.
„
Chapter 5
Briefly provides commentary on Reference Loads and the effects on road wear.
„
Chapter 6
Summarises the effect on road wear from vehicle weight distribution across axle
groups.
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„
Chapter 7
Briefly comments on the effect on road wear from vehicle types.
„
Chapter 8
Summarises the main points in respect to road wear from vehicle loading and
utilisation.
„
Chapter 9
Briefly summarises views on the effect of large single tyres on road wear.
„
Chapter 10
Discusses the appropriateness of a diesel excise, in respect to road wear.
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2.
The New Zealand Road User Cost (RUC)
System
This section briefly describes the current RUC model in respect to vehicle wear effects. Figure 1 Overview of NZ RUC, shows the overall outline of the current RUC model.
From Figure 1 it can be seen that the actual costs are allocated to the different road vehicle types
and these costs are recovered through the Road User Charges.
„
Figure 1 - Overview of NZ RUC1
Cost Allocation
Vehicle Fleet Effects
Expenditure items in National Land Transport Programme
RUC vehicle numbers and VKT
Non‐RUC vehicle numbers and VKT
Cost allocation parameters
Cost allocation proportions
Total cost
per cost allocation parameter
Deduct fixed revenue
Total number of units for each cost allocation parameter
Unit rates for each cost allocation parameter
Charges per kilometre for each vehicle type and weight
Equivalent fuel excise for non‐RUC vehicles
Road User Charges
1
Heavy Vehicle Road User Charges Investigation, February 2008, TERNZ COVEC
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2.1.
Road costs and vehicle weight and axle loading
The theoretical relationship between road construction and maintenance costs, vehicle weight and
axle loading is based on the use of fatigue laws. The fatigue law is a mathematical relationship
between a road pavement structural strength and the number of allowable load repetitions.
The fatigue damage model is considered through Miner’s hypothesis which assumes that each axle
load application “consumes” a part of the life of the pavement equal to the inverse of the allowable
load repetitions.
The key is to determine an equivalence between the different axle loads and axle groups such that
the cumulative damage can be estimated and compared to the allowable load repetitions.
This is where the Fourth Power rule was developed to enable different axle groups and axle loads
to be converted to an Equivalent Standard Axle (ESA).
The Fourth Power Rule is discussed further in Chapter 3, as to its origins and its applicability to
New Zealand roads.
The ESA or “wear effect” of a vehicle is calculated in the CAM using the following:
Wear = (Maximum Gross Weight/Sum of Axle Reference Loads)4 x Number of Axles x 0.552
Where the Axle Reference Load depends on:
„
•
the number of axles;
„
•
the number and arrangement of tyres on each axle, ie the axle type;
„
•
tyre sizes, or more specifically the tyre contact areas; and
„
•
axle spacing.
Additionally the “wear” formula above assumes through the 0.55 factor, that the vehicles will be
loaded about half (55% of the time) the time.
Table 1 below shows the Axle Reference Loads for standard tyres and various spacing.
2
Review of the Road User Charges Cost Allocation Model, Report to MoT, Allan Kennaird Consulting 29
January 2007
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„
Table 1 - Axle Reference Loads For Standard Tyres (tonnes/axle)
Single Tyred
Twin Tyred
Four Tyred
Eight Tyred
Spaced axles
6.70
8.20
13.00
14.90
In close group of
two axles
7.05
8.62
13.70
15.69
In close group of
three axles
7.17
8.77
13.91
15.94
In close group of 4
or more axles
7.26
8.88
14.08
16.14
Hence for a 3 axle truck, single steer and twin driven axle with 21 gross weight the “wear” using
the above formula is :
= (21/(6.2+23.92))^4 x 3 x 0.55 = 0.98 ESA
In the following chapters we review the impacts of:
•
Reference Loads;
•
Vehicle Loading and Utilisation;
•
Vehicle Weight Distribution; and
•
Large Single tyres.
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3.
The Fourth Power Rule
3.1.
Introduction
The Fourth Power Rule was mainly based on performance data derived from the American
Association of State Highway and Transportation Officials (AASHTO) Road Test (Highway
Research Board, 1962).
Some key features and findings of the trials were:
„
„
Conducted in Ottawa, Illinois between 1958 and 1961;
The temperature range was between 24.5oC in summer and -2.8oC in winter, and with average
rainfall of 864mm;
„
Used various pavement sections with flexible pavement section consisting of various
thicknesses of asphalt surfacing, basecourse , subbase and subgrade;
„
Some sections had asphalt surfacing of 25mm which is equivalent to NZ thin road surfacings;
„
All had the same subgrade with a value of CBR 3;
„
Failure was defined through a Present Serviceability Rating (PSR) which was defined as “ The
judgement of an observer as to the current ability of a pavement to serve the traffic it is meant
to serve”;
„
The PSR had a scale of: (0–1) very poor, (1–2) poor, (2–3) fair, (3–4) good, and (4–5) very
good;
„
Using regression analysis the subjective PSR could be correlated to Present Serviceability
Index based on physical road wear measurements such as roughness , cracking, patching and
rutting;
„
From the analysis they developed Load Equivalence Factor (LEF) which is basically the
number of axles passing a point to generate the same pavement wear as the standard axle (a
standard axle is defined as 8 tonnes);
„
The LEF derived from AASHTO are complex and depend upon axle load, axle configuration,
pavement type, pavement strength (structural number) and the value of failure as defined by
PSR;
„
LEF could be expressed as:
LEFPSI = N0 / Nx | PSI
where:
„
N0 : Number of 80-kN single axle loads to produce a limiting value of PSI
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„
„
Nx : Number of repetitions of selected axle configuration (single or tandem) of load x
to produce the same limiting value of PSI
In 1970, Scala observed that the AASHTO LEFs derived from above equation are
approximately equal to the fourth power of the ratio of the actual loads.
LEFPSI = (Lx/L0)4 where:
Lx : Arbitrary axle load
L0 : Standard axle load (18 kips for single axles, 30 kips for tandem axles)
3.2.
Review of Recent NZ Research
The most recent research in New Zealand on the LDE was conducted at CAPTIF between 2000 and
2004. The following reports were the output of the research:
„
Report No. 214, by Arnold et al., 2001: Prediction of pavement performance from repeat load
tri-axial tests on granular materials;
„
Report No. 207, by de Pont et al., 2001: Effect on pavement wear of an increase in mass limits
for heavy vehicles [Stage 1];
„
Report No. 231, by de Pont et al., 2002: Effect on pavement wear of an increase in mass limits
for heavy vehicles – Stage 2;
„
Report 279, by Arnold et al., 2005a: Effect on pavement wear of increased mass limits for
heavy vehicles – Stage 3;
„
Report 280, by Arnold et al. 2005b. Effect on pavement wear of increased mass limits for
heavy vehicles – Stage 4; and
„
Report 281, by Arnold et al. 2005: Effect on Pavement Wear of Increased Mass Limits for
Heavy Vehicles.
The CAPTIF facility is circular track where a rotating arm moves in a circular direction mimicking
the loadings that can be applied to a pavement to assess the relative damaging effect i.e. the LDE.
The CAPTIF is capable of having different loads applied to the pavement in different wheel paths.
The comments in this section are based on the final concluding Report 281.
Table 2 shows the different test segments, base course materials, subgrade and relevant reports
applicable to those tests.
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„
Table 2: CAPTIF Test Segments and attributes3
Pavement
Segment
ID
Pavement
Thickness
(mm)
Gravel
Source
Description
Report
Cptf_A03
320
Montrose
Cptf_B03
250
Class 2
20mm max size
rhyolite from
Montrose, Victoria,
Australia
Arnold et al.
(2005a) [Stage
3, RR279]
Cptf_C03
250
Cptf_D03
320
AP40 TNZ
M/4
Cptf_E03
320
Rounded
AP40
TNZ M/5
40mm max size
alluvial gravel,
greywacke from
Canterbury, NZ
40mm max size
uncrushed rounded
river gravel from
Canterbury, NZ
Cptf_A01
300
AP40 TNZ
M/4
40mm max size
alluvial greywacke
gravel from
Canterbury, NZ
Cptf_B01
300
AP40 TNZ
M/4
40mm max size
alluvial greywacke
gravel, deliberately
contaminated with
10% by mass silty
clay fines from
Canterbury, NZ
+ fines
Cptf_C01
300
Montrose
Class 2
Cptf_D01
300
Recycled
concrete
Test 40 kN
compared to
60 kN
Subgrade
Silty clay
CBR=11%
de Pont et al.
(2001)
[RR207]
Test 40 kN
compared to
50 kN
20mm max size
rhyolite from
Montrose, Victoria,
Australia
Recycled crushed
concrete from
Auckland building
demolition sites
As with all research not all combinations and variables can be studied at the same time in the
available timeframe, hence certain assumptions had to be and made some key ones are:
„
The “vertical surface deformation (VSD) was used as the main measure of pavement wear”3;
„
The end of life of pavement is defined as “the number of wheel passes when 10% of the area
has a VSD greater or equal to 15 mm”3; and
„
The pavement base course material was placed on an identical subgrade for all test segments
and it “was a silty clay with a CBR of 11%,”3.
3
Report 281 by Arnold et al. 2005: Effect on Pavement Wear of Increased Mass Limits for Heavy Vehicles
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3.2.1.
Vertical Surface Deformation
The use of VSD as a surrogate to rutting and roughness (longitudinal unevenness) is sound
although there are some basic differences which the report recognises.
Firstly in the CAPTIF test it is shown that VSD is greater than rutting as can be seen in the Figure
2.
„
Figure 2: Comparison of VSD and rut measurement at CAPTIF3
It is not clear from the report as to why this is the case apart from the statement that the “for this is
related to the initial reference level taken for calculating the VSD values and how the edges of the
ruts at CAPTIF move downwards during the testing which affected the straight-edge rut depth”3.
From the above figure it can be suggested that rutting is generally 20% less than VSD, this is
important as it can affect the comparison to the existing road network and other accelerated
pavement loading trails, eg ALF and AASHTO.
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3.2.2.
End of life criteria
The definition adopted is “when 10% of the area has a VSD greater or equal to 15 mm”3, this is
justified by considering six factors of which the 3 that are of concern are:
„
The level of rutting has no impact in determining the LDE;
„
This level of rutting is currently applied to the state highway network in NZ; and
„
By adopting a medium level of rutting as end of life then there is less likelihood of the need to
extrapolate the results as compared to adopting a worst level of rutting (this would indicate
time was limited for the research project and hence a low value was chosen).
The first assumption in the report is backed by the figure below. It is stated that “Figure 2.3 shows
the marginal effect of the chosen VSD value on the damage law exponent"3.
However when examining the above figure it is clear that in 3 of the 4 segments the LDE increases
as the VSD measurement increase. This change is significant i.e. for a 30% increase in VSD there
is a 10% increase in exponent value. This is not unexpected as the ASSTO trials showed for
different end of life definitions i.e. PSI the load equivalencies factors for the same axles groups
were different.
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When considering the end of life as being 15mm of VSD on 10 percentile of the segment as being
representative of current practices is it important to reflect that the RUC wear component is applied
to actual costs and not theoretical costs so a direct comparison to actual pavement performance is
warranted.
The 2007 Condition report4 states with regards to rutting that “The network, shows signs of
deterioration, with 0.51% of the network having rutting > 20mm compared with 0.41% last year,
but this is still within Transit’s SOI target of < 1%”. See figure below from the 2007 Condition
Report4.
Hence the failure / end of life is considered to be 20mm of rutting but there is no indication of 10%
of the segment, it is simply a percentage of the network i.e. length of network which exceeds 20mm
of rutting.
Assuming the 20% difference from Figure 2: Comparison of VSD and rut measurement at
CAPTIF3is correct, then an end of life of 20mm of rutting translates to 24mm of VSD
measurement, which is far different from the 15mm adopted in the CAPTIF trails. This difference
is important as it has an impact on the LDE derived.
4
Transit New Zealand State Highway National Pavement Condition Report 2007
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The other key consideration in choosing the VSD of 15mm was to pick a vertical displacement low
enough such that the end of life is reached during the testing phase. This would negate the need to
post-extrapolate the results to determine the number of passes at the end of life stage.
It is clear from the past research i.e. ASSHTO, that the defined terminal level of distress has an
influence of the number of cycles and the LDE derived.
Even though a low VSD level was chosen for end of life definition the majority of the test
segments required to be post-extrapolated. This is another avenue for introducing errors in the final
conclusions.
In comparison during the recent Accelerated Loading Facility (ALF)5 trials in Australia of unbound
granular pavement with thin surfacing failure was “defined as 25 mm maximum mean surface
deformation” or “until the deformation reached such a magnitude that trafficking was no longer
possible”. This is more consistent with road practitioners’ definition and of note is the use of the
mean measurement and not the 10 percentile of the segment.
3.2.3.
VSD Extrapolated to end of life
The researchers considered two methods to extrapolate the VSD measurement to 15mm to estimate
the end of life of the pavement segments. They were principally linear and a power model. Figure
3 shows how the two models were applied to one station reading.
5
AP-T104/08 Relative Pavement Wear of an Unbound Granular Pavement Due to Dual Tyres and Single
Tyres, by Richard Yeo, Siew Leng Koh, Kieran Sharp, August 2008
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„
Figure 3: Linear and power model extrapolation methods compared to actual results
The report concludes that even though the power model is a better fit to the data, that the linear
model is more appropriate for thin surfaced pavements.
The impact of which model is chosen for extrapolating the VSD has a significant influence on the
outcome of the LDE. The report states that using the power model as compared to the linear model
results “in a higher damage law”3.
„
Figure 4: Extrapolation of VSD data using the power model
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„
Figure 5: Extrapolation of VSD data using the linear model
By examining Figure 4 and Figure 5 it clearly shows the impact of using the different
extrapolation models on the LDE. Note the difference between 60kN and 40kN for power and
linear in terms of the number of wheel passes.
This difference is further reinforced by Table 3 and Table 4 below.
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„
Table 3: LDE using Linear model for extrapolation to end of life
Pavement Life in Wheel passes (106) –
Extrapolated by Linear model
LDE Report
281
Average
10th %ile
90th %ile
10th %ile
ID
Load
(KN)
Cptf_A03
40
2.9
2.4
3.4
Cptf_A03
60
1.7
1.3
2
Cptf_B03
40
3
2.8
3.3
Cptf_B03
60
1.4
1.2
1.6
Cptf_C03
40
2.8
2.4
3.2
Cptf_C03
60
1.4
1.1
1.5
Cptf_D03
40
3.2
2.6
3.8
Cptf_D03
60
1.5
1.2
1.9
Cptf_E03
40
0.8
0.4
1.4
Cptf_E03
60
0.2
0.1
0.3
Cptf_A01
40
4.5
2.5
7.7
Cptf_A01
50
2.3
1.1
3.4
Cptf_B01
40
4.8
2.4
7.8
Cptf_B01
50
2.4
1.3
3.4
Cptf_C01
40
6.4
4.5
8.3
Cptf_C01
50
4.7
2.2
7.5
Cptf_D01
40
6.2
4.3
9.4
Cptf_D01
50
4.2
3.4
5.1
1.1
Average ->
2.3
1.5
2.0
2.0
1.9
3.2
3.4
2.7
3.2
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„
Table 4: LDE using Power Model for extrapolation to end of life
Pavement Life in Wheel passes (10^6) - Power
ID
Load
(KN)
LDE
calculated
from adjoining
figures by
SKM
Average
10th %ile
90th %ile
90th %ile
Cptf_A03
40
5.7
3.9
8.2
Cptf_A03
60
1.9
1.4
2.2
Cptf_B03
40
7
5.7
8.2
Cptf_B03
60
1.5
1.3
1.8
Cptf_C03
40
6.1
4.6
9.5
Cptf_C03
60
1.7
1.2
2
Cptf_D03
40
11.9
5.2
16.3
Cptf_D03
60
2
1.4
2.6
3.2
3.7
3.8
4.5
Cptf_E03
40
0.5
0.2
0.8
Cptf_E03
60
0.2
0.1
0.3
Cptf_A01
40
41.9
4.1
135.5
Cptf_A01
50
4.7
1.3
9.6
Cptf_B01
40
62.7
3.6
89.6
Cptf_B01
50
10.1
1.8
22.3
6.2
Cptf_C01
40
72.1
14.5
213.1
Cptf_C01
50
88.8
2.6
226.3
This segment
data was
ignored as the
number of
passes for
40KN is less
than 50KN in
the 90th
percentile
which is
counterintuitive.
Cptf_D01
40
85.6
18.8
211.8
Cptf_D01
50
14.1
7.4
23
9.9
Average
5.7
2.4
11.9
From the above tables it is shown that the linear extrapolation method leads an average LDE of 2.3,
whereas a power model extrapolation on the 90h percentile data results in an average LDE of 5.7.
Furthermore the Report 281 states that during the development of a deterioration model based on
material properties from Repeated Load Triaxial (RLT) that “The extrapolation method used was a
power model as this provided the best fit”3. It is clearly inconsistent to use different extrapolation
methods in the same research project.
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3.2.4.
Subgrade strength
The research at CAPTIF limited the CBR value to 11% for all subgrades in all segments. This
subgrade is relatively high and would contribute significantly to the pavement strength.
Clearly the strength of the subgrade has had an impact on the pavement strength and this is
recognised in the research report.
From Table 5 a pavement with 300mm of basecourse similar to the CAPTIF pavement trails and a
weaker subgrade CBR of 5% there is a 3 fold increase in the LDE. Clearly the subgrade
contributes to the pavement strength and has a greater impact on thin weak pavements as compared
to thicker pavements.
„
Table 5: Effect of subgrade on SNP and LDE3
Pavement Thickness &
Subgrade CBR
SNP (estimated)
LDE(derived from
pavement model)
300 mm/CBR10%
3.5
1.5
300 mm/CBR5%
2.8
4.8
700 mm/CBR10%
4.9
1.1
700 mm/CBR5%
4.3
0.9
3.2.5.
Summary comments on Report 281
In reviewing the Report 281 with reference to the 4th power rule the following can be summarised:
„
The LDE can be shown to be between 2.9 and 11.9 if you adopt a Power Model to extrapolate
the VSD until the end of life instead of the Linear Model adopted in the report;
„
Assuming that the 10th percentile of the segment as the basis for measurement is not based on
any pavement condition reported by the roading authorities;
„
The VSD measurement in CAPTIF is actually a higher measurement than rutting. Hence a
VSD of 15mm for terminal stage of a pavement is equivalent to 11mm which is well below the
actual level used and reported by roading authorities being 20mm of rutting;
„
The strong subgrade contributes to the pavement strength and in turn reduces the power law
exponent;
In concluding there does not seem to be sufficient evidence to change the power rule from the 4th
to a lower level.
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3.3.
Review of other relevant reports
3.3.1.
Ternz and Covec6
This report was commissioned by the Ministry of Transport NZ. Amongst its objectives was a
requirement to obtain further information on the “fourth power rule for allocating axle weight
related roading expenditure to vehicle use”.
Their literature search is present in Table 6.
„
Table 6: LDE from various researchers
Pavement type
Test conditions
Exponent
Reference
Flexible
Modelling with validation
1.3-4.1
(Cebon 1999)
Flexible
Modelling using AASHO results
2-6
(Addis and Whitmarsh 1981)
Rigid
Reported values from OECD
members
8-12
(OECD 1982)
Rigid
Reported values
11-33
(Cebon 1999)
Thin surface
granular
Accelerated pavement testing in
Australia
1.8-5.9
Thin surface
granular
Accelerated pavement testing in
New Zealand
1.3-3
(Yeo, Sharp et al. 2004)
(Martin 2005)
(de Pont 2001; de Pont 2002; Arnold
2005; Arnold 2005; Arnold 2005)
In summary the report’s literature search suggests that:
„
No universal fourth power relationship exists and that there are many variables at play;
„
The exponent value was dependent upon pavement type, the pavement strength, the type of
distress chosen for end of life determination, for example rutting compared to roughness, the
terminal value chosen, and the analysis methodology.
3.3.2.
Arnold et al.7
The CAPTIF research concluded amongst other items:
„
6
“The exponent for the power law ranged from 2 to 4 for Segments A, B, C and D.”;
Heavy Vehicle Road User Charges Investigation, February 2008, TERNZ COVEC
7
Effect on pavement wear of increased mass limits for heavy vehicles – stage3, by G. Arnold, B. Steven, D.
Alabaster & A. Fussell, Land Transport New Zealand Research Report 279,2005
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„
“The value of the exponent depended on the pavement type and the value of VSD taken to
be the end-of-pavement life.”
„
“The result of this accelerated pavement test principally provides an indication of the
performance of a relatively strong pavement, on a strong dry subgrade, in ideal dry
environmental conditions. The behaviour of weaker or saturated subgrades has not been
investigated, nor have the effects on older and/or poorly maintained surfaces where
moisture may be entering the base.”
So the findings above are no different to the ASSHTO trials in that they confirm:
„
3.3.3.
the Exponent is dependent upon the terminal / failure definition, that strong pavement
provide values of 2 to 4 in ideal conditions and that it would not be unexpected to have
higher exponent values for weaker pavements.
Sharp and Vuong8
Reported on ALF test trails conducted to test thin surfacing on unbound crushed rock pavement
typical of pavements used in arterial roads (i.e. total pavement thickness of 200 mm on a subgrade)
they conclude “The load damage exponent (LDE) value determined for this pavement was about
7.5, which is similar to the LDE values of 6-8 estimated from a previous ALF trial, much higher
than the LDE of 4 obtained from the AASHO Road Test, but similar to the “subgrade damage”
exponent of 7.14 recommended in the Austroads Pavement Design Guide”.
3.3.4.
Yeo, Martin and Koh9
They reported on ALF trials of unbound granular material with thin surfacing as shown in Figure
6. Three materials were tested including New Zealand M4 material.
8
Characterisation and Axle Load Equivalency of Unbound Granular Pavements by K.G. Sharp and B.T.
Vuong 2000
9
Investigation of the Load Damage Exponent of Unbound Granular Materials under Accelerated Loading,
AP-T73/06, by Richard Yeo, Tim Martin and Siew Leng Koh, 2006
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„
Figure 6: Typical pavement structure
Their analysis of the results is shown in Table 7.
„
Table 7: LDE
Material Deformation Roughness CC blend (50% / 50% blend of recycled
crushed concrete and quarry product) 3.2 4.8 Montrose (high quality material - an Australian
rhyolite / rhyodacite, crushed rock) 3.8 3.4 NZ M4(high quality gravel- a New Zealand
crushed alluvial gravel) 2.0 ‐ One of their key conclusions is:
“Overall it was considered that the 4th power law (LDE = 4) was adequate for the types of
pavements studied and under the test conditions”.
3.3.5.
Chen et al10
This research paper outlines the performance of thin surfaced pavements in an instrumented fullscale test pavements when built inside a temperature-moisture controlled environment, and
subjected to accelerated traffic by means of a heavy vehicle simulator (HVS).
10
Analysis of Overload Damages in Thin Flexible Pavements by Chen, Dar-Hao ; Cortez, Edel R; Petros,
Katherine A; Zhou, Fujie ; Yang, Wei-Shih Transportation Research Board Annual Meeting 2006 Paper
#06-0391
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A LDE was found to be higher than 7.8, they believe that the high exponent values obtained were
attributable to low structural capacity (SN) of the test pavements.
They consider that the well known 4th-power rule is too conservative for low SN pavements.
This is in line with NZ research which shows that low strength pavement will have an exponent
value greater than the fourth power rule.
3.3.6.
Johnsson 200411
Johnsson examined other researcher’s data and stated that for flexible pavements the fourth power
law seems to be justified.
3.3.7.
De Pont 12
De Pont reviewed the methodology for calculating the exponent by Arnold et al. in the Report 281
and found “some inconsistencies in the method used by Arnold et al to determine the power law
exponent in the pavement deterioration model. This does not necessarily mean that all Arnold’s
findings are wrong. However, those involving the power law exponent cannot be relied upon.”
3.3.8.
Austroads13
The Austroads pavement design guide recognises the different failure mechanism and suggests
different damage exponents for each pavement type and failure mode. Table 8 shows values from
4 to 12.
11
.The cost of relying on the wrong power – road wear and the importance of the fourth power rule, by
Johnsson, R. 2004, Transport Policy 11 Issue 4, October 2004:
12
Methodology for Calculating the Exponent in a Pavement Wear Model, by John de Pont, TERNZ Ltd,
February 2008
13
Austroads 2004 Pavement Design Guide
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„
Table 8: LDE from Austroads Pavement Design Guide
Design
method
Pavement type
Damage type
Damage
designation
Damage exponent
m
k
Empirical
(Figure 8.4)
Mechanistic
3.4.
Granular
pavement with thin
bituminous
Surfacing
Overall pavement damage
e
4
Pavement
containing one or
more bound layers
Fatigue of asphalt
a
5
Rutting and shape loss
s
7
Fatigue of cemented materials
c
12
Fourth Power Rule Conclusions
The fourth power rule has been used extensively around the world and there has been significant
body of research into the robustness of the rule. It can be concluded that the research points to the
following:
„
There is no unique value of the LDE for all pavements and this is recognised in current
Austroads pavement design guide;
„
All research confirms the existence of damage power relationship with a standard 8 tonne axle;
„
Most research indicates that the value of the power exponent varies with:
–
The pavement type i.e. weak or strong, and rigid or flexible;
–
The subgrade strength;
–
The environmental factors which will influence the pavement strength;
–
The type of distress chosen for the definition of end of life egg rutting compared to
roughness, or the PSR used in the original AASHTO trials;
–
The terminal value chosen;
–
The method of analysis of the data; and
–
The material properties of the pavement.
With regards to New Zealand’s recent research results from the CAPTIF trials it can be concluded
that:
„
The tests were done on specific pavement types (strong pavements) and the results cannot be
applied to the entire road network of national and local roads;
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„
The definition of terminal value adopted of 15mm VSD (which is about 20% less than rutting
measurement in the CAPTIF setup) seems on the low side as compared to road practitioners
definition i.e. 20mm of rutting;
„
The use of the 10 percentile measurements as the terminal value is not in line with other
accelerated pavement trials such as the ALF which uses the mean value;
„
The majority of the pavement segments did not fail and had to have their VSD values postextrapolated using a linear relationship even though they state that the power relationship has a
better data fit. This has a significant effect on the reported values from an average exponent
value of 2.3 for linear model extrapolation to 5.7 for power model extrapolation;
„
The predictive modelling calibrated against the CAPTIF results show that for a weaker
subgrade (CBR =5%) in lieu of CBR of 11% in the trials that there is dramatic increase in the
exponent value, from 1.5 to 4.8; and
In conclusion from the above research there is insufficient robust and clear evidence to change the
RUC model for road wear by changing the fourth power rule. To apply a different LDE over
different parts of the network would be complex and administratively difficult to implement.
Changing the fourth power rule to a lower power value over the entire New Zealand road network
would be a risky strategy, and one in our view which should be avoided.
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4.
Vehicle Dynamics and Its Effect on Road
Wear
4.1.
RUC Model
The vehicle dynamics has an important effect on pavement deterioration due to the dynamic loads
that can be generated from stiff spring suspension systems.
From the previous section the “wear” formula does not include any explicit factors for dynamic
loading of heavy vehicles. However the Fourth Power rule implicitly includes this dynamic
loading effect as it is derived from field trials in the early sixties where the heavy vehicle
suspensions would have been stiffer than today’s vehicles.
4.2.
Effect on Road wear
The summary of research shows:
„
The measured dynamic wheel forces vary with suspension type, road roughness and vehicle
speed;
„
On moderately rough roads vehicles travelling at highway speed and poorer performing
suspensions can have up to double the wheel loads than those of the better suspensions
systems;
„
On smooth roads there is less difference between suspension types hence the specification for
new roads works to have low roughness values;
„
General road-friendly suspensions systems have low vertical stiffness and high levels of
dampening;
„
Air suspensions tend to have better dynamic performance than mechanical suspensions but
their maintenance is vital to ensure continuity of their road friendliness;
„
Air suspensions with poor damping can produce higher levels of dynamic loading as
mechanical suspensions; and
„
As most heavy vehicle suspensions have similar stiffness they tend to respond in a similar
manner when they encounter a particular road profile and this is exhibited in a spatial
repeatable pattern causing localised pavement failure rather than a general pavement wear.
Currently in Australia and in other EU countries there is recognition of the different suspension
effects by allowing higher mass vehicles for road-friendly suspensions. In Australia and in
particular in NSW there is a required for the transport operator to have a maintenance quality
system to ensure the road friendliness of the suspension system before they are allowed the higher
mass.
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4.3.
Other Views
The BCA believes that airbag suspensions should obtain a discount as:
“Most buses and coaches have airbag suspension which reduces road wear, but the RUC scale,
developed in the late 1970s has failed to recognise these improvements”
OPUS 1999 recommends in their report that “alternative sets of RUC rates for vehicles fitted with
road-friendly suspensions” be developed.
4.4.
Conclusion
SKM is of the opinion in keeping with Australian practice, that the effects of road-friendly
suspensions should be recognised by a new vehicle type in recognition of their road friendliness,
provided the suspension system is monitored through a third party system.
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5.
Reference Loads
5.1.
RUC Model
The RUC model assumes a “wear” ESA as discussed in Chapter 2. The maximum gross weight of
the vehicle is divided by the reference load as part of that formula. In other words the Reference
Load is the base line from which other loads are compared against to estimate their relative road
wear effects.
The RUC Reference Loads are shown in Table 1 - Axle Reference Loads For Standard Tyres
(tonnes/axle).
5.2.
Effect on Road wear
The Reference Load reflects the ESA of the different axles and axles groups which is directly
related to the cumulative ESA that a pavement can withstand during it service life.
5.3.
Other Views
It has been suggested that the Reference Loads used in the cost allocation model for determining
the number of ESA associated with a particular axle should be the same as those values used in the
Austroads pavement design guide. This guide is used in the design and rehabilitation of pavements
in New Zealand.
The Austroads design loadings are shown in the Table 9: Austroads axle group reference loads for
pavement design.
„
Table 9: Austroads axle group reference loads for pavement design
Axle group type
Load (kN)
Single axle with single tyres (SAST)
53
Single axle with dual tyres (SADT)
80
Tandem axle with single tyres (TAST)
90
Tandem axle with dual tyres (TADT)
135
Triaxle with dual tyres (TRDT)
181
Quad-axle with dual tyres (QADT)
221
Hence, consider a 3-axle truck (RUC type 6) at 21 tonnes gross weight. It will have 6 tonnes on the
steer axle and 15 tonnes on the drive axle group.
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„
Under RUC pavement wear model the steer axle generates 0.64 ESA and each drive axle
generate 0.57 ESA, giving a total of 1.78 ESA;
„
Whereas using the Austroads Reference Loads (Table 9), the steer axle generates 1.52 ESA
while the drive axle group generates 1.41 ESA, giving a total of 2.93 ESA.
It would seem from the above simple example that the RUC Reference loads are less conservative
than the Austroads Reference loads.
5.4.
Conclusion
SKM is of the opinion that the Reference Loads in the RUC model should be changed to align with
Austroads as they are currently being used in pavement design. A consistent approach should be
taken.
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6.
Vehicle Weight Distribution
6.1.
RUC Model
The RUC model assumes that the loads are equally distributed between all the axles as evidenced
by the “wear” formula and the Reference Loads are per axle.
In the above example (Chapter 5) of the 3-axle truck (RUC type 6) with 6 tonnes on the steer axle
and 15 tonnes on the drive axles, the respective ESAs are 0.64 ESA and 0.57 ESA. Whereas the
RUC model assumes each axle to be 0.59 ESA giving a total of 1.77 ESA as compared to
1.78 ESA.
This difference is small but does increase slightly as the vehicle is partially loaded as is shown by
the example below:
„
6.2.
Assume 3-axle truck (RUC type 6) with 5 tonnes on the steer axle and 12 tonnes on the drive
axles the respective ESAs are 0.31 ESA and 0.235 ESA. Whereas the RUC model assumes
each axle to be 0.25 ESA giving a total of 0.75 ESA as compared to 0.78 ESA.
Effect on Road wear
Calculating the right ESA is important to road wear as an increase in ESA increases the road wear
over the lifetime of the pavement.
6.3.
Other views
It has been suggested that this averaging out of ESA per axle in the RUC model is disadvantages to
certain vehicle types. In particular, powered vehicle loads on the front do not increase with
payload as much as the loads on the drive axles, however trailers the loading is more proportional.
Hence assuming “uneven load distribution into account will lead to proportionately more RUCs
being attributed to powered vehicles and less to trailers particularly at lower levels of loading.”
6.4.
Conclusion
SKM is of the opinion that the manner in which the RUC model currently treats the load
distribution is not as significant as the other issues raised in this report. The impact is marginal and
other more influential factors (vehicle dynamics and reference loads) should be addressed first.
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7.
Vehicle Types
7.1.
RUC Model
The RUC model classifies the vehicle fleet into different vehicle types depending upon the number
of axles configurations. Almost 50 percent of RUC vehicle types have a one to one relationship to
the vehicle configuration. The remaining vehicle types have a one to many vehicle configurations.
7.2.
Effect on Road wear
The vehicle types have an applicable ESA based on the axle configurations, spacings and number
of tyres. Hence if the vehicle type includes several combinations of vehicles then this could lead to
under or overestimating the ESA of that vehicle type.
7.3.
Other Views
The TERNZ Covec report suggests:
„
RUC type 24 possible “one vehicle type for twin-tyred single axle trailers and one for all other
single axle trailers”;
„
RUC type 28 – “included there is need to subdivide this vehicle type”;
„
RUC type 37 – “There should be separate vehicle types for these two configurations”; and
„
RUC type 43 – “certainly a case for separating the vehicles with twin tyres from those with
wide singles in the RUC schedule and probably a case for separating the semitrailers from the
full trailers”.
7.4.
Conclusion
The data is improving with greater capability of developing additional vehicle types at minimal
administrative cost. It is SKM’s opinion that new vehicle types should be investigated to better
represent the current vehicle fleet.
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8.
Vehicle Loading and Utilisation
8.1.
RUC Model
The RUC model assumes a factor of 0.55 to reflect that half the distance travelled (55%) is at
maximum gross weight and the other half (45%) is at tare weight. Hence when the vehicle is
empty it is assumed to have the wear effect of 1/10 of a full vehicle.
8.2.
Effect on Road wear
Clearly the impact of underestimating the proportion of vehicles loaded can have a direct impact on
estimating the ESA applied to the pavements.
8.3.
Other views
The TERNZ Covec report analysed the WIMs data and found that the assumption in the RUC
model are: “Analysis shows that with the current CAM method for calculating ESA a 55% factor
for average ESA is reasonable for powered vehicles but the factor for trailers should be 44%.”
The Mckenzie Podmore 2008 report14 points out that some freight operators have specific
operations which the average might not be applicable such as:
„
“Logging trucks and tankers that are configured for a single commodity with zero backhaul
prospects,
„
Vehicles engaged in round trip collection and distribution services where maximum weights
are achieved for less than half the journey
„
Vehicles with full backloads
„
Normally lighter vehicles that operate at a constant loading all the time because there is little
difference between the tare (or tare plus machinery) and maximum weight.
Failure to recognise these different operational practices or dealing with them on the basis of
national averages introduces another source of cross-subsidy into the CAM and undermines the
efficiency of vehicle configuration decision making as well as the overall integrity of the user-pays
system.”
14
Efficiency and Equity Issues in the Funding of Roading Expenditures – May 2008 Mckenzie Podmore
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The BCA suggests:
“Buses tend to operate at capacity for less than 15% of the time in contrast to freight vehicles on
which the 55% loading rate was determined. Bus and coach tare weights are also higher relative to
freight.”
furthermore
“Bus or coach unladen weights (tare) typically represent around 70% of the total permissible fully
laden weight”.
8.4.
Conclusion
In the absence of other evidence it is SKM’s opinion that the 0.55 factor be maintained until better
classification between RUC vehicle type and WIM data is available.
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9.
Large Single tyres
9.1.
RUC Model
The RUC axle reference loads assume standard tyres. The RUC model takes into account large
single tyres through the gross contact area of the tyre, which is obtained from tyre manufacturer's
data.
However the RUC rate schedules do not differentiate between standard tyres and wide single tyres
in identifying vehicle types. The classification of vehicles into RUC types is sole based on axle
configuration and number of tyres, but not tyres type.
9.2.
Effect on Road wear
The replacement of twin standard tyres with large single tyres increases the stress on the pavement
and hence the ESA. For example the relative damaging effect of single axle fitted with large tyres
as compared with dual tyres is (8.2t)4/(7.2t)4 = 1.68, an increase of 68%.
9.3.
Other views
TERNZ Covec report suggests that “RUC vehicle type classification groups some vehicle
configurations together into a single type where it would be more appropriate to distinguish
between them because their pavement wear characteristics are quite different”.
OPUS 199915 recommended “no change in the RUC model to make allowance for vehicles fitted
with single large tyres”. This is based on Table 10: Axle Reference Loads for Large Tyres
Compared with those used in Current RUC Calculations, which shows minor differences and where
difference exist then the proportion of vehicles are small.
15
New Zealand’s Roading Cost Allocation Model Review of Engineering Issues 17 September 1999 Opus
International Consultants Allan Kennaird Consulting
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Table 10: Axle Reference Loads for Large Tyres Compared with those used in Current
RUC Calculations
„
RUC Vehicle Type No.
5
6
28
37
43
Sum of Reference Loads currently
used in RUC calculation
22.8
23.8
14.1
25.0
34.52
Sum of Reference Loads for single
large tyres
22.9(1)
24.4(2)
15.2(3)
23.1(4)
30.3(5)
(1)
Single large tyres on rear tag axle in close group with twin tyred axle, single standard tyres on front axle
Single large tyres on front axle and twin tyred rear axles in a close group
(3)
Tandem close group of axles both fitted with large single tyres
(4)
Tridem close group of axles all fitted with large single tyres
(5)
Two close groups of axles all fitted with large single tyres
(2)
9.4.
Conclusion
SKM is of the opinion that single wide tyres should have different vehicle type classification and a
corresponding RUC rates schedule.
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10. Diesel Excise
It has been suggested that the use of diesel excise can be used to replace the current RUC method
as it does in Australia. This section explores the engineering concepts and validity of such a
proposal.
A research report conducted into investigating ways of saving fuels for heavy vehicles16 concluded
“Weight is one of the most essential factors affecting fuel consumption. The dead weight of trailers
and vehicles should be minimised. An extra 1,000 kg in weight, either as dead weight or as load,
adds some 0.7 l/100 km in fuel consumption for a truck-trailer combination in highway driving. In
dynamical urban driving the comparable value for buses is some 2 l/100 km per 1,000 kg”.
Hence the application of a diesel excise as a substitute RUC for road wear will not be applied
equitably across the heavy duty fleet and network i.e. urban situations with stops and starts will pay
a higher level.
Furthermore the study shows how the fuel consumption increases linearly as the load increases, see
Figure 7.
„
Figure 7: The fuel consumption of heavy duty vehicle as loads increase16
16
Finish VTT research centre - Fuel savings for heavy-duty vehicles. Summary report 2003 – 2005. English
version.
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This linear relationship does not match the expected road wear as a consequence of the increase
vehicle weight, which is an exponential relationship. A simple illustration is provided by the
Automobile Association’s submission as shown in Figure 8.
„
Figure 8: RUC vs. Excise for Class 6
It is clear from the above graph that there is no relationship between fuel consumption of heavy
vehicles and the RUC charges and in particular the road wear charges. As the fundamental
mathematical relationship of a linear model for fuel consumption and a power model for road wear
damage cannot coincide.
Figure 9 shows the RUC charges for the different vehicle types in RUC. Applying a diesel excise
which is linear would then favour the heavy duty vehicles at the expense of the light vehicles.
But worst is the fact that high axle loads cause more damage than pavement friendly axle groups
which spread the load on the pavement. Currently the RUC charges take care of this impact but a
flat diesel excise would simply encourage heavier loads per axle.
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„
Figure 9: RUC Charges
In summary a diesel excise as a replacement for RUC road wear charges would result in:
„
Heavier vehicles would pay less than currently under the RUC regime and lighter vehicles
would pay more even though they contribute less pavement damage;
„
The RUC vehicle classification recognises less damaging axle group classification through the
CAM whereas a diesel excise does not;
„
Vehicles that are used on private roads would require a rebate scheme of the diesel excise
thereby increasing the administrative cost;
„
Non road activities egg marine, rail, construction and mining would similarly require a rebate
scheme of the diesel excise;
„
Vehicles which use fuels other than diesel egg trolley buses, hybrid buses and bio diesel buses
would not be covered by the diesel excise and would need some sort of RUC charges to
recover their contribution towards of road maintenance and construction costs.
Fundamentally a diesel excise is not related to pavement wear whatsoever.
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Appendix A List of References
i.
Report 281 by Arnold et al. 2005: Effect on Pavement Wear of Increased Mass Limits for
Heavy Vehicles
ii.
Transit New Zealand State Highway National Pavement Condition Report 2007
iii.
AP-T104/08 Relative Pavement Wear of an Unbound Granular Pavement Due to Dual Tyres
and Single Tyres, by Richard Yeo, Siew Leng Koh, Kieran Sharp, August 2008
iv.
Heavy Vehicle Road User Charges Investigation, February 2008
v.
Effect on pavement wear of increased mass limits for heavy vehicles – stage3, by G. Arnold,
B. Steven, D. Alabaster & A. Fussell, Land Transport New Zealand Research Report
279,2005
vi.
Characterisation and Axle Load Equivalency of Unbound Granular Pavements by K.G.
Sharp and B.T. Vuong 2000
vii.
Investigation of the Load Damage Exponent of Unbound Granular Materials under
Accelerated Loading, AP-T73/06, by Richard Yeo, Tim Martin and Siew Leng Koh, 2006
viii. Analysis of Overload Damages in Thin Flexible Pavements by Chen, Dar-Hao ; Cortez, Edel
R; Petros, Katherine A; Zhou, Fujie ; Yang, Wei-Shih Transportation Research Board
Annual Meeting 2006 Paper #06-0391
ix.
The cost of relying on the wrong power – road wear and the importance of the fourth power
rule, by Johnsson, R. 2004, Transport Policy 11 Issue 4, October 2004:
x.
Methodology for Calculating the Exponent in a Pavement Wear Model, by John de Pont,
TERNZ Ltd, February 2008
xi.
Austroads 2004 Pavement Design Guide
xii.
Finish VTT research centre - Fuel savings for heavy-duty vehicles. Summary report 2003 –
2005. English version.
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